One main theme is friendship. Since they are both strong - willed characters from different universes, their friendship is often explored. Another theme is cooperation. They usually have to work together to face various challenges.
I think 'Eternal Companions' is a great Cloud x Leon fanfiction. It starts with a chance encounter between them. As the story progresses, they discover they have more in common than they thought. The author does an amazing job of developing their relationship gradually. There's also 'Fusion of Souls' which delves deep into their psyche and how they come to understand and trust each other on a very profound level.
Cloud is often brooding and a bit of an outsider, while Leon is more stoic and duty - bound. In a fanfic, their first meeting could be filled with tension as Cloud's more impulsive nature clashes with Leon's methodical approach. For example, when planning a battle strategy, Cloud might want to rush in, while Leon would prefer to wait and gather more information.
One idea could be that Cloud and Leon are from different dimensions. They meet by some strange magic accident. Cloud is on a journey to find his lost memories, and Leon, being a more experienced fighter in his world, helps him understand the true nature of his past. They bond over their shared experiences of fighting powerful foes.
I'm not sure about a specific one off the top of my head. But you can check out some popular fanfiction websites. They usually have a search function where you can type in 'leon x mr x' and browse through the results. Look for ones with high ratings and lots of reviews.
Hunnigan could be a great addition. She could provide Leon with information and support from the outside, and her presence could add some tension as she might be suspicious of Ada's motives. Another good character is Wesker. His past connection to Ada could create more drama and conflict in the story. He could be a shadowy figure trying to manipulate the situation between Leon and Ada.
One interesting aspect could be the unique relationship dynamic they might have. Maybe Leon is constantly on the run from Mr. X, and the fanfiction could explore the cat - and - mouse chase in a more in - depth way, like how Leon outsmarts Mr. X in different scenarios.
One common theme is friendship. They often start from different backgrounds or even with some animosity, but gradually become friends through shared experiences.
Romance is also a big theme. Writers like to develop a romantic relationship between them, showing how their feelings for each other might have grown during their time together in the game's story. They might write about secret glances and unspoken feelings that gradually turn into love.
One common theme is adventure. Since both Leon and Sherry are characters from the Resident Evil universe which is full of danger and mystery, many fanfictions will have them go on adventures together, fighting against bio - horrors. Another theme is romance, of course. Writers like to explore the relationship between the two, how they might fall in love and support each other.